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Roiling magnetic waves explain solar enigma

By David Shiga

Video: Cloud-like structures in the background above the Sun churn from the passage of magnetic ripples called Alfvén waves, which may be responsible for the extreme heat of the Sun’s corona. Vertical jets called spicules dance closer to the Sun’s surface (Courtesy of Science)

Magnetic waves ripple through the Sun’s outer atmosphere with enough energy to heat the region to its astonishing temperature of millions of degrees, new views from the Hinode spacecraft suggest. If correct, the waves could solve a decades-long puzzle about the source of this heat.

Temperatures in the Sun’s wispy outer atmosphere, or corona, are in the millions of degrees, even though temperatures at the Sun’s surface are only a few thousand degrees. Scientists have puzzled for more than 50 years about why the corona is so hot.

Some scientists have suggested that the required energy is carried up to the corona by magnetic ripples called Alfvén waves. The idea was bolstered by recent observations by a ground-based telescope that revealed for the first time that such waves are present in the corona. But these observations were not detailed enough to determine whether the waves carried enough energy to make the corona as hot as is observed.

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Now, ultra-high resolution observations from Japan’s Hinode spacecraft suggest the waves really do carry enough energy to heat the corona, a team of scientists says. The team was led by Bart De Pontieu of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California, US.

The Alfvén waves are undulations of the magnetic field, which is invisible. But the waves can in principle be detected by the way they buffet things that are visible. The team believes that motions of jets called spicules that emerge like whiskers from the Sun’s surface are due to Alfvén waves.

Swaying forest

The observations show the jets are not rooted to one spot but drift back and forth horizontally. The characteristic speed and frequency of the spicules match the results of computer simulations where Alfvén waves move upward through a forest of spicules and cause them to sway back and forth.

The detailed motion of the spicules seen by Hinode allowed the team to calculate that if the swaying really is due to Alfvén waves, they carry at least 120 watts per square metre into the corona, more than enough to maintain the corona’s tremendous heat. Scientists are still debating different ideas for how the Alfvén wave energy could be converted into heat. The amount of energy in the waves also makes them a good candidate for powering the streams of charged particles that move outward from the Sun, called the solar wind.

The calculations are an important step forward for understanding heating in the corona, according to Robertus Erdelyi and Viktor Fedun, both of the University of Sheffield, UK, who are not members of De Pontieu’s team.

“For the first time it has been clearly shown that the energy associated with the waves is more than enough to accelerate the solar wind and heat the quiet corona,” they write in a commentary in the journal Science. It is not yet clear whether the waves can account for even hotter temperatures in active regions of the corona, where strong magnetic fields create structures such as coronal loops.

Other observations from Hinode reported in the journal include&colon;

 Massive flows of plasma from areas of vigorous activity at the Sun’s surface up into the corona. There, it may then leave the Sun altogether, accounting for up to 25% of the material in the solar wind

 Separate observations of huge jets of plasma up to 20,000 kilometres wide and 100,000 kilometres long in the Sun’s corona, possibly driven by Alfvén waves, which may also contribute to the solar wind